University of Montana ScholarWorks at University of Montana

Graduate Student Theses, Dissertations, & Professional Papers Graduate School

1983

Kinematic interpretation of mylonitic rocks in Okanogan dome north-central Washington and implications for dome evolution

Vicki L. Hansen The University of Montana

Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y

Recommended Citation Hansen, Vicki L., "Kinematic interpretation of mylonitic rocks in Okanogan dome north-central Washington and implications for dome evolution" (1983). Graduate Student Theses, Dissertations, & Professional Papers. 7442. https://scholarworks.umt.edu/etd/7442

This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. COPYRIGHT ACT OF 1976

Th is is an unpublished m anuscript in which copyright sub ­ s i s t s . Any further r e p r in t in g of it s contents must be approved BY th e a u th o r .

Ma n s f ie l d L ibrary Un iv e r s it y of Montana Date : 1 9 R f t

KINEMATIC INTERPRETATION OF

MYLONITIC ROCKS IN OKANOGAN DOME,

NORTH-CENTRAL WASHINGTON,

AND IMPLICATIONS FOR DOME EVOLUTION

by

Vicki L. Hansen

B. A., Carleton College, 1980

Presented in partial fulfillment of the requirements for the degree of

Master of Science

UNIVERSITY OF MONTANA

1983

Approved by

Chair, Board of Examiners

D ^ , Graduate School

Date UMI Number: EP38243

All rights reserved

INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted.

In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMT Dissertation Publishing

UMI EP38243 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code

ProQ^sf

ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346 ABSTRACT

Hansen, Vicki L ., M. S., May 1983 Geology

Kinematic interpretation of mylonitic rocks of the Okanogan dome, north-central Washington, and implications for dome evolution

Director: Donald W. Hyndman

Mylonitization and later brecciation of granodiorite record the environment and timing of ductile and b rittle deformation in the Okanogan dome. Deformation and high-grade metamorphism of sed­ imentary rocks was accompanied and followed by emplacement of granodiorite plutons. The paragneiss and granodiorite were mylon- itized, then warped and domally uplifted with coincident develop­ ment of joints, dikes, and chloritic breccia. The myloni te zone dips welst to southwestward and is about 1.5 km thick. Mylonitization increases structurally upward within the dome. Quartz, biotite, and plagioclase record a mylonitic folia­ tion (S) containing a unidirectional elongation lineation. Super­ imposed shear surfaces (C) cut the foliation at an angle of 10- 4 5 0 , the angle decreasing with increasing mylonitization. The angle of inclination of S to C indicates a sense of westward displacement, of upper plate rocks relative to lower plate rocks, in a direction parallel to mylonitic lineation. This sense of shear is opposite to the shear indicated by quartz c-axes and reoriented folds (Goodge, 1983) and stretched inclusions. The mylonitic fabric formed isochemically under middle-green- schist conditions as determined by plotting the percent Ab compon­ ent in coexisting recrystal.lized feldspars. A zone of intense chloritJc breccia disrupts the mylonitic layer­ ing along the west and southwest border of the dome. The zone, about 30 m thick, is subparallel to the mylonitic foliation and confined to the dome margin. Clay gouge, siickensides, and polished surfaces are prominent within the breccia zone. I t is conceivable that the Okanogan mylonite zone experienced earlier easterly directed shear as indicated by reoriented folds and stretched inclusions, followed by later westerly directed shear as indicated by the S and C asymmetry. The ductile mylonitic deformation was followed by more-recent movement which caused brecciation of the mylonitic layering.

n ACKNOWLEDGEMENTS

I am indfebted to Dave A lt, Brain Atwater, Ken Fox, John Goodge,

Don Hyndman, Dean Rinehart, and Steve Sheriff for liv e ly discussions in the fie ld and many helpful suggestions. I extend special thanks to John Goodge and Don Hyndman for insights, encouragement, and patience. I accept full responsibility for the (mis)interpretation within this report, and thank Carol Simpson and Gordon Lister for their challenging remarks which opened my mind to the "truth". This work was accomplished with generous support from the U. S. Geological

Survey and the Colville Confederate Tribes, and through research grants from the Geological Society of America (2990-02) and Sigma Xi.

m TABLE OF CONTENTS

Page

ABSTRACT...... ii

ACKNOWLEDGMENTS...... i i i

LIST OF FIGURES AND TABLES...... V

INTRODUCTION ...... 1

GENERAL GEOLOGY ...... 7

MYLONITIC FABRIC ...... 8

Fabric Interpretation ...... 13

Megacryst Patterns...... 23

CHEMISTRY...... 30

Feldspar Mineralogy ...... 32

B iotite Chemistry ...... 35

CONCLUSIONS...... 40

TECTONIC FRAMEWORK ...... 41

REFERENCES CITED ...... 48

IV LIST OF FIGURES AND TABLES

Figure Page

1. Metamorphlc core complexes of the North American Cordillera . .2

2. Location map of the Okanogan dome, north-central Washington . .2

3. Generalized geologic map of Omak Lake 15* quadrangle ...... 4

4. Biotite schistose inclusion in megacrystic granodiorite. . . 10

5. Block diagram of mesoscopic mylonitic fabrics...... 10

6. Photomicrographs of mylonitic textures ...... 15

7. Generalized sketch of Mm and M...... e 21

8. Block diagram of microscopic mylonitic fabrics...... 21

9. Sketch of Me and Mm with interpreted sh e a...... r 24

10. Models of feldspar megacryst deformation...... 24

11. Model of feldspar megacryst r o ta tio...... n 24

12. Comparision of Mm and Me with lit e r a t u r e...... 28

13. Comparision of Mm and Me with S and C ...... 28

14. Comparision of Mm and Me with Sc and Sm...... 28

15. Ternary diagram of feldspar Ab, An and Or components...... 34

16. Graph of percent Ab in coexisting recrystal 1ized feldspar . . 34

17. Ternary disgram of Fe, Mg and Ti in Me and Mm biotite . . . .3 7

TABLE

I. Rotation of feldspar megacrysts...... 3 7

I I . Whole rock major element analyses...... 38

I I I . Plagioclase mineralogy and chemistry...... 38

IV. Orthoclase mineralogy and chemistry...... 38

V. Coexisting feldspars ...... 39

VI. B iotite mineralogy and chemistry...... 39

V ADDENDUM

In the following report I present a logical arguement for a top-to-the-east sense of shear during the formation of the Okanogan dome mylonitie zone, north-central Washington. Although the argue­ ment appears logical and internally consistent, it is based on the assumption that the major shear plane parallels the mylonitic f o l i ­ ation (Me), marked by compositional layering (page 27). If Me, as I describe i t , formed by stretching RATHER THAN by major shear, my interpretation of the mylonitic fabrics is incorrect. Mm planes would then describe the major sense of shear during formation of the mylonite zone, and hence the shear described by the asymmetry of the Me and Mm planes i f top-to-the-west. Based on the orientation of quartz subgrains this reinterpreted top-to-the-west sense of shear appears correct. Elongate quartz subgrains, interpreted as qualitative strain ellipsoids, are titled to the west and inclined at an acute angle to Mm. Mm planes then become the interpreted planes of shear which deformed the resultant quartz subgrains (Fig. A). The shear on Mm planes is le f t la te ra l, or top-to-the-west, in a direction parallel to mylonitic lineation. Given this revised interpretation my Me planes correspond to S planes mylonitic schistosity, of Berthe et al (1979), and my T-Yn planes correspond to C planes, shear planes, of Berthe et al (1979). Read cautiously and critically dear reader--my arguement sounds logical and internally consistent. Unfortunately/fortunately the Okanogan story is not yet as simple as top-to-the-east versus top-to-the-west. Although the petrotextures of the megacrystic granodiorite seem to now be telling us top-to-the-west, stretched inclusions (Fig. 4), and reoriented metamorphic folds and quartz c-axes (Goodge, 1933) describe a top- to-the-east sense of shear within the mylonite zone. It may be that the Okanogan mylonitie formed during earlier easterly directed shear as described by macroscopic inclusions and reoriented folds, and later experienced westward directed shear reflected on a smaller scale in the Mc-Mm asymmetry.. .but why then the easterly biased quartz c-axes? As any good study this research asks more questions than i t answers.

W

Fla. A

VI INTRODUCTION

Okanogan, Kettle, and Spokane domes in northeastern Washington extend south from the Shuswap metamorphic complex of south-central

British Columbia (Fox et a l, 1977; Cheney, 1980; Rhodes and Cheney,

1981). These crystalline domes along with at least 22 others form a sinuous chain along the North American Cordillera from southern Canada to northwestern Mexico (Fig. 1) (Davis and Coney, 1979). They have attracted considerable attention as metamorphic core complexes (see

Crittenden et al, 1980). The salient features of Cordilleran meta­ morphic core complexes include a high-grade metamorphic-plutonic infrastructure deformed by a gently-dipping mylonite zone overprinted by a zone of chloritic breccia. Above the breccia zone suprastructural rocks are metamorphosed as high as sillim anite zone of the amphibolite facies although greenschist-grade is most common. The role of the mylonitic deformation in dome genesis remains controversial. In this paper I describe the mylonitic rocks and their textures, and conclude that their fabric formed isochemically under middle-greenschist facies conditions as rocks above the mylonite zone were displaced eastward relative to dome rocks. This conclusion is based on the percent albite component in coexisting recrystallized feldspars (Stormer, 1975) and an interpretation of textural asymmetry within the mylonitic fabric similar to that of Berthe et al (1979).

Okanogan dome consists of 2,500 square kilometers of metasedimentary

1 riTTEPFOOT OOr-'E

Ok A'iOGA'i 'JO'IE

300 km

FIG. 1. Location of metamorphic core complexes along the North American Cordillera from Canada to Mexico (adapted from Davis and Coney, 1979).

119" 110 117'

Osoyoos

KETTLE of OKANOGAN DOME / I DOME Omak

map a re a Spokane—^P^SPOkANE >i HO lA / DOME /

iO m.

FIG. 2. Location map of the Okanogan dome, northeastern Washington Omak Lake 15’ quadrangle is shown in the southwest portion of Okanogan dome. (Adapted from Cheney, 1980). gneiss and massive granitoid gneiss cut by Eocene age granitic plutons

(Fox et a l, 1976). Okanogon dome is separated from Kettle dome to the east by Tertiary volcanic rocks and clastic terrigenous sedimentary rocks of the Republic Graben; rocks of Okanogan dome are nearly con­ tinuous to the north with high-grade rocks of the Shuswap metamorphic complex (Fig. 2). Tertiary basalts of the Columbia Plateau end abruptly at the southwest dome boundary (Fig. 3). Rocks within the dome record Eocene K-Ar ages (Fox and Rinehart, 1971). Low-grade metamorphosed sedimentary rocks of Triassic age or older, flank the dome to the north and northwest (Fox et a l, 1971, 1976, 1977). High- grade paragneiss and associated plutonic rocks flank the dome to the south and southwest (Goodge and Hansen, in press; B. Atwater, pers. comm.). Okanogan dome, forming the western-most boundary of the

Omineca crystalline belt (Fox et a l, 1976), is bounded to the west by the Okanogan fau lt and to the south by the Omak Lake fau lt (Snook,

1965; Goodge and Hansen, in press). An extensive, well-developed mylonite zone, up to 1.5 km thick, penetrates rocks of the crystalline core along the northern, western, and southern borders of the dome

(Waters and Krauskopf, 1941; Snook, 1965). The mylonitic fabric, consisting of a fo liatio n and unidirectional lineation, dies out in the dome interior (Fig. 3). A randomly fractured breccia cuts the mylonitic fabric along the western and southern dome boundaries

(Waters and Krauskopf, 1941; Snook, 1965; McMillen, 1981; Hansen,

1983, Goodge and Hansen, in press). L 19*^30 ■ ^ 119° 1 5 '

48 ° 3 0 '

OMAK LAKE 15 QUADRANGLE 0 1 2 3 4 5 ^ MI GEOLOGY CY ^ 0 i Z 3 4 5 J. /J. GOOUGE a n d 7 . L . HAiibEN CM O CM CD KM

FIG. 3. Generalized geologic map of the Omak Lake 15' quadrangle, Okanogan County, Washington. MAP SYWOLS

lithologie contacts olgd Omak Lake K-feldspar megacryst biotite granodiorite Light to medium-grey biotite granodiorite contains chloritic breccia zone up to 5% K-feldspar megacrysts. Equigranular matrix contains quartz, plagioclase, biotite and minor sphene and Fe oxide. Unit is cut by pegmatite and ^ ^ __ _ synform showing trace of axial plane aplite dikes and contains tabular to rounded inclusions and plunge of axis of amphibolite and biotite schist.

^ ^ anti form, showing trace of axial plane ^ bptl Bull Pasture hornblende-biotite tonalité and plunge of axia Dark grey to black, medium-grained, equigranular, locally layered, spotted tonalité. Unit is Intruded by olgd and Ipg. ^ lim it of mylonitic deformation; mylonitized rock on hatchered side of line-gradational boundary egd Equigranular (hornblende-) biotite granodiorite Light-grey, medium-grained, massive, equigranular granodiorite contains minor hornblende, sphene, strike and dip of mylonitic foliation apatite, epidote and chlorite. Cl, hornblende: biotite ratio and sphene content commonly increase near contact with pgn. Contact zone is 0.5 km wide, trend of mylonitic lineation becoming wispy to migmatitic.

mgd K-feldspar megacryst biotite granodiorite Coarse-grained, massive to foliated, blue-grey rock contains K-feldspar megacryst (l-6cm). Flow fo l­ I~Q~I Quaternary Deposits iation parallels mafic inclusions. Unit is cut by alaskite, and aplite and pegmatite dikes. I Tb } Columbia River Basalt

I Tv I Coleman Butte volcanics Tonasket Gneiss (usage after Fox et a l, 1976) Undivided assemblage of high-grade gneisses including hornblende-biotite gneiss, biotite gneiss, augen gneiss, Ipg Luecocratic porphyritic granite sillimanite-garnet-muscovite-biotite schist, biotite White to grey, medium- to coarse-grained quartzite, amphibolite, diopside calc-silicate gneiss, porphyritic granite. Contains pegmatite and and garnet alaskite gneiss. Unit is intruded by olgd apalite dikes. Weak mylonitic fabric occurs and fvgd to the south. locally along south end of Omak Lake. Ipg intrudes bpt along the southeastern margin of Omak Lake. pgn Paragneiss [ Undivided assemblage of high-grade, compositionally layered gneisses including hornblende-biotite gneiss, fvgd French Valley biotite granodiorite augen gneiss, biotite gneiss, sillimanite-muscovite- Light-grey, medium-grained, equigranular biotite schist, biotite-quartzofeldspathic gneiss, biotite granodiorite with minor hornblende, amphibolite, quartzite, biotite quartzite, marble, sphene, apatite, and Fe oxide. Contacts with (SI garnet-pyroxene cala-silicate gneiss, coarse hornblendite, olgd gradational over 50 m but probable and minor metamorphic (?) dunite. Unit is intruded by zenoliths of olgd suggest fvgd is younger. mgd and egd. Geologic relations observed along the southwest border of Okanogan dome constrain several models presented by ea rlier workers attempting to explain the origin of the rocks and their structures. Waters and

Krauskopf (1941) originally attributed both the mylonitic and cata- clastic textures to protoclastic emplacement of the "Colville batholith" defined by Pardee (1918) (see Fox et a l, 1976, p. 217-219). Snook

(1965) reexamined western and southwestern sectors of the dome and concluded that parent rocks of the core gneiss, the Tonasket gneiss, are sedimentary and volcanic rocks regionally metamorphosed to high- grades. According to Snook the gneiss was subsequently cut by a deep- seated, low-angle thrust fault which produced a mylonitic fabric. The area was then raised along the Okanogan Valley and Omak Lake faults causing brecciation and retrograde chloritization. These normal faults placed low-grade metamorphic rocks against high-grade core gneiss to the east. Snook's interpretation accounts for the unidirectional mylonitic lineation, and the very different environments during for­ mation of mylonite and breccia.

Fox et al (1971, 1976, 1977) studied the area further; they identified and redefined a centrally located body of paragneiss, the

Tonasket gneiss, and a discontinuous peripheral body of gneissic to massive granitoid rocks. These workers disclaim Snook's late-stage normal faults and attribute the penetrative deformation and meta­ morphism of dome rocks to mobilization and diapiric intrusion of core rocks into low-grade country rocks. Fox et a l, like Waters and

Krauskopf, attribute both the mylonite and breccia to diapiric emplacement of a crystalline core. Cheney (1980) suggests that thrusting and cataclasis separated a basement of Precambrian meta­ morphic rocks and Mesozoic to Tertiary plutons from Precambrian and

Tertiary layered rocks. Folding and doming followed during Eocene time.

This paper discusses the southwest border of the Okanogan dome, and stems from geologic mapping in the Omak Lake 15’ quadrangle

(Fig. 3). My research involves fie ld mapping and laboratory study,

including analysis of microscopic fabrics and microprobe study of samples collected in three traverses through the mylonite zone. The objective is to constrain models of mylonite formation, and hence dome evolution.

GENERAL GEOLOGY

Within the Omak Lake 15' quadrangle, Okanogan dome is bounded to the west by the Okanogan fa u lt, and to the south by the Omak Lake fault (Snook, 1965). An extensive and well-developed mylonite zone, up to 1.5 km thick, penetrates the dome rocks along the northern, western and southern borders of the dome, and is overprinted by a narrow zone of ch loritic brecciation along the southwest border within the study area. Rocks outside the southwest margin are divided into four groups: high-grade layered metasedimentary gneiss, felsic granitoid rocks, mafic to felsic dikes, and Tertiary basalt. Rocks within the southwest portion of the dome also comprise four major lithologie units: high-grade layered metasedimentary gneiss, homogeneous 8

orthogneiss, leucocratic porphyritic granite, and microdiorite dikes.

All units inside the dome, with the exception of microdiorite, are deformed by varying degrees in the mylonite zone. Pre-mylonite geologic histories of rocks inside and outside Okanogan dome are strikingly similar. Both packages of rock record deposition of eugeoclinal sediments followed by high-grade regional metamorphism and deformation to form paragneiss sequences displaying strong meta­ morphic textures. Folding occurred during or after the peak of regional metamorphism. A sequence of granodiorite to tonalité plutons intruded the paragneiss assemblages, followed in turn by intrusion of leucogranite. At this point the geologic histories of these two rock packages diverge. The rocks of Okanogan dome were deformed by pene­ trative, unidirectional mylonitization, followed by more restricted ch loritic brecciation along its borders. Rocks west of the dome margin are not mylonitized, yet they record brecciation and chloritization similar to b rittle deformation along the dome margin. A deformation which accompanied and followed mylonitization warped the mylonite zone into gently plunging, megascopic, anti forms and synforms. North­ east-trending vertical joints formed in the dome and microdiorite dikes subsequently intruded along these jo in t planes (see Goodge and

Hansen, in press).

MYLONITIC FABRIC

My research focuses on mylonitized megacrystic granodiorite in order to determine the environment of mylonite formation. Orthogneisses, exposed in a broad band northeast of Omak Lake, cover the greatest portion of the study area (Fig. 3). Three major compositional phases are recognized, K-feldspar-megacryst biotite granodiorite, equigranular biotite granodiorite and hornblende-biotite tonalité, all of which contain numerous pegmatite and aplite dikes, and amphibolite and biotite-schist inclusions. Megacrystic granodiorite (olgd) is a lig h t- to medium-gray-colored (Cl = 5-7), homogeneous-textured plutonic rock containing conspicuous pink or grey K-feldspar mega­ crysts 2-8 long cm) comprising 1 to 5 percent of rock volume. Its matrix is medium-grained (0.5-5.0 mm) and equigranular, and contains quartz, plagioclase, biotite, and minor orthoclase, sphene, allanite, and opaque Fe-oxide. Megacrystic granodiorite is in sharp contact

(—10 m wide) with tonalité and equigranular granodiorite. However, the three units are probably contemporaneous phases of a single pluton as indicated by xenocrysts of K-feldspar megacryst in equigranular granodiorite and tonalité near their contacts with megacrystic granodiorite, inclusions of megacrystic granodiorite in equigranular granodiorite, and wide dikes of megacrystic granodiorite cutting equigranular granodiorite.

Dominating the form and structural fabric of the rocks within

Okanogan dome is a penetrative zone of mylonite, as defined by Bell and Etheridge (1973). This broad zone conforms to the dome perimeter, and forms "fla t-iro n "-lik e aprons slanting up eastward from the Okanogan

Valley as well as the gentle domal shape of the Okanogan highlands.

The mylonite ranges in thickness from 1.0 to 1.5 km, and intensity 1 0

FIG. 4. Biotite schistose inclusion in megacrystic granodiorite; inclusion is viewed northward in a plane parallel to mylonitic lineation and normal to mylonitic foliation.

N 3 0 Ç , ~N60y

5" cm I Norma I to M e FIG. 5. Block diagram of mesoscopic mylonitic fabrics. Note that 11

of deformation increases upward in the dome. Mylonitic deformation

increases toward the western and southern dome boundaries. I t is well defined in the orthogneiss near border areas of Okanogan dome.

The mylonitic foliation is a broadly planar biotite and quartz fabric, which on a grain scale is wavey or sigmoidal in coarse-grained ortho­ gneiss. In the mylonitic foliation plane, elongate lenses and streaks of aggregate biotite, quartz and plagioclase define a unidirectional

lineation. Strikes of the mylonitic foliation grossly parallel the

dome boundary, but lineation bearings fa ll consistently in the ranges

140°-155°.

Three mesoscopic features of the mylonitic structures indicate

important aspects of the physical conditions during deformation;

1) relative orientations of pegmatite dikes within the zone, 2) de­

formed inclusions, and 3) pull-apart fractures in K-feldspar-megacrysts

Planes of numerous pegmatites and mafic dikes which intrude K-felds-

par megacryst granodiorite show increased concordance to mylonitic

fo liatio n upward in the mylonite zone. This pattern is a consequence of increased deformation toward higher structural levels in Okanogan dome, and reflects not only greater shear strain but greater overall

tectonic transport toward the margin of the dome. Biotite-rich

schistose inclusions are elongate with an upper tail stretched east­ ward and a lower ta il westward, as viewed on a plane normal to mylonitic foliation and parallel to mylonitic lineation (Fig. 4). The

pattern of these inclusions suggests dextral shear in the mylonite

zone, viewed northward. In K-feldspar megacryst granodiorite. 12

megacrysts are cut by planar brittle fractures oriented perpendicular to mylonitic lineation. The fractures end abruptly at crystal boundaries, do not cut into mylonitic fo liatio n , appear unrelated to crystal orientation, and commonly contain a quartz-chlorite- epidote mineral assemblage. Formation of the fractures must be a response to mylonitization and not a post-mylonitic period of brittle deformation, because they cut only megacrysts, not the mylonitic fabric. Such pull-apart fractures also indicate that mylonitization involved local extensional strain.

To determine the environment of mylonite formation, I collected rock samples at seven or more stations along each of three traverses through progressively mylonitized homogeneous megacrystic grano­ d io rite, and studied trends in mineral and chemical composition, and textures (Fig. 3). I collected oriented samples for thin section and microprobe mounts at every station along each traverse, and samples for whole-rock chemical analysis and specific gravity at every other station along each traverse. Sections for petrofabric and microprobe analysis are consistently oriented with mylonitic foliation and lineation as depicted in Figure 5. Sections cut normal to mylonitic foliation. Me, and parallel to mylonitic lineation. Ml, are viewed northward; sections cut normal to Me and Ml, are viewed westward.

Sections from traverse A were used for microprobe analysis. 13

FABRIC INTERPRETATION

Mylonitic deformation of megacrystic granodiorite produced an asymmetric penetrative fabric consisting of several mesoscopic and microscopic fabric elements. The most striking mesoscopic fabric

is mylonitic foliation and lineation. Layers or lenses rich in quartz

interspersed with layers of biotite and feldspar define a well-

developed planar compositional mylonitic fo liatio n . Me. Within the

foliation plane, streaks of biotite, quartz, and plagioclase define

a unidirectional mylonitic lineation. Ml. In the Omak Lake 15'

quadrangle, this lineation trends approximately 150*. Blocky

elongate K-feldspar megacrysts commonly display quartz-filled, sub­

vertical fractures oriented normal to Ml (Fig. 5). In sections cut

normal to Me and parallel to Ml, a weakly- to moderately-developed

crenulation cleavage. Mm, crosses the mylonitic foliation, defining

a fabric asymmetry. The crenulation cleavage. Mm, is variably de­

veloped, striking parallel to mylonitic foliation. Me, and dips more

steeply westward 15-45* (Fig. 5). The intersection of these two

planes is normal to Ml. Aligned and smeared biotite grains coat the

Mm plane defining its role as a slip surface. These biotite grains mark a second lineation. Ml, within the plane of Mm nearly parallel

to Ml, though plunging more steeply westward. On surfaces cut normal

to both Me and Ml, a single planar foliation continuous with Me is

present. Mm is best developed high in the structural section where

the mylonitic fabric is strongest. In the fie ld . Me is the dominant

fo liatio n ; Me is planar and continuous whereas Mm resembles 14

chattermarks or unidirectional scalloped surfaces 1 to 2 cm across cutting Me surfaces at a consistent angle. The planar continuity of

Me, and accompanying strong lineation Ml, suggest that Me and Ml are the plane and direction of major tectonic transport.

Similar fabrics and relationships appear in thin section. Layers of alternating quartz and feldspar lenses define mylonitic foliation.

Me, most obviously in plane lig h t. In sections normal to Me, and parallel to Ml, quartz lenses form sigmoidal layers which thicken and thin asymmetrically between planar enveloping surfaces parallel to mesoscopic Me (Fig. 6 .a). Elongate quartz subgrains observed in polarized light show aspect ratios of 4:1 to 10:1 roughly parallel to the outline of the sigmoidal quartz lenses. Greater ratios structurally high in the mylonite zone reflect increased mylonitic deformation.

In sections normal to Me and parallel to Ml, slip surfaces Mm

intersect Me at positions along the thinned limbs of quartz in Me

(Figs. 6.b and 7). Mm surfaces are makred by biotite and other mafic minerals, and zones of ductile grain-size reduction of feldspar and stretching or elongation of quartz (Figs. 6.b and 6.c). Biotite forms stringers of short, very fine-grained ( 35um) biotite film along Mm planes. The angular intersection of Me and Mm defines a textural asymmetry in sections parallel to Ml and a symmetrical planar foliation in sections parallel to the Mc-Mm intersection

lineation in sections normal to Ml (Fig. 8). The significance of Me and Mm asymmetry observed in sections normal to Me and parallel to Ml,

is discussed below. Fig. 6.a. Layers of alternating quartz and Fig. 6.b. In sections normal to Me and feldspar lenses define mylonitic foliation, parallel to Ml, slip surfaces Mm Intersect Me, best seen In plane lig h t. Section Is horizontal Me at an acute angle. Mm surfaces cut normal to Me and parallel to Ml; field are marked by biotite and other mafics, and of view Is 8.5 mm. zones of ductile grain-size reduction of feldspar and elongation of quartz.

Fig. 6.e. Mm planes owe their short length Fig. 6,d. Mm rarely truncates Me toatlly; to truncation by movement along Me planes as rather It deflects Me reflecting minor shear evidenced by biotite In this photomicrograph along Mm. Feldspar megacryst offset along Mm which Is 3.5 mm across. The fine-grained shows measurable le ft-la te ra l displacement biotite stringer marking Mm Is rotated Into viewed northward. Field of view Is 3.5 mm. Me Indicating right-lateral movement along Me.

LO i 4

I t 1 Fig. 6.C, The angular intersection of Me and Mm define a textural asymmetry in sections parallel to Ml. Me is horizontal with Mm cutting at an angle of 3Q0. Note the sigmoidal nature of Me; the enveloping surface of microscopic Me parallels mesoscopic Me. Field of view is 8.5 mm.

Fig. 6.g. In sections cut normal to both Fig. 6.f. In sections parallel to Ml, Me and Ml no textural asymmetry is developed, megacrysts display imbrication , titled Megacrysts are generally rounded and show no eastward relative to Me which is horizontal preferred orientation, or they are elongate Field of view is 3.5 mm. parallel to horizontal Me and show no imbrication. Field of view is 8.5 mm.

Fig. 6.h. In this photomicrograph, 3.5 mm Fig. 6.i. This fractured feldspar megacryst across, each sucessive feldspar fragment indicates overall clockwise rotation denoted shows left-lateral offset parallel to Mm. by the arrows. Me is horizontal with Mm dipping Me is horizontal. 30*^ to the left. Field of view is 8.5 mm.

CT»

2 1

Me Me Mm

FIG. 7. A generalized sketch of microscopic fabrics show Mm in te r­ secting Me at positions along the thinned limbs of quartz in Me. Section is cut normal to Me and parallel to Ml. The planar enveloping surface parallels Me in hand speciman.

N30E

M l NbOW,

nor ma cm

FIG. 8. Intersection of Me and Mm results in imbrication and fabric asymmetry in sections cut normal to Me and parallel to Ml. Sections cut normal to both Me and Ml lack imbrication and fabric asymmetry. 22

Mm planes cross Me planes along the thinned quartz limbs of Me.

Elongate quartz subgrains of Me are deformed along Mm planes in­ dicating left-lateral offset as shown in Figure 6.b). Mm rarely truncates Me totally; rather it deflects Me reflecting minor shear along Mm. Feldspar megacrysts offset along Mm also show similar small, measurable le ft-la te ra l displacement viewed northward (Fig. 6.d)

Therefore, mesoscopic smeared-bio tite lineation and offset micro­ structures indicate Mm planes are shear surfaces with slip normal to the Mc-Mm intersection. Mm surfaces show consistent le ft-la te ra l offset viewed northward in sections parallel to Ml throughout the studied structural thickness of the mylonite zone.

Although Mm planes deform Me, Mm planes are short and discon­ tinuous relative to Me. Mm planes owe their short length to truncation by movement along Me planes as evidenced by the biotite in Figure 6.3.

In this photomicrograph the fine-grained biotite stringer marking Mm

is rotated into Me indicating right lateral movement along Me. This relationship is typical of Mc-Mm-plane interaction. Whereas Mm planes offset or deflect Me planes. Mm planes also shallow or merge with Me planes with the geometry of lis tr ic normal faults. In summary, both

Me and Mm planes were loci of shear, or movement. Microstructural evidence indicates that at least a portion of the movement histories overlapped in time with relative apparent offset on each surface pictured in Figure 9; viewed north in sections parallel to Ml, Me and

Mm planes indicate right-lateral and le ft-la te ra l displacement

respectively. A kinematic interpretation of the interaction of Me and 23

Mm surfaces is discussed below following the discussion of mega­

cryst geometry.

MEGACRYST PATTERNS

Feldspar megacrysts deflect the planar Me and Mm surfaces.

Megacrysts show kinking, bending, fracturing, and breakage indicative

of dominantly b rittle deformational processes and the rigid nature of

feldspar in a ductile matrix (Lister and Price, 1978; Watt and

Williams, 1979). In sections parallel to Ml, megacrysts display

imbrication, tilted eastward relative to Me (Figs. 5 and 6.f). This

megacryst imbrication is an additional component of the textural

asymmetry in the mylonitic fabric. In sections normal to Me and Ml

megacrysts appear generally rounded and show no preferred orientation,

or they are elongate parallel to Me and show no imbrication (Figs. 5

and 6.g).

The imbricate nature of the feldspar megacrysts is portrayed in

Figure 6.h. Note the apparent le ft-la te ra l offset between several

successive feldspar fragments. The resulting domino-like geometry

can result through two different movement models (Fig. 10). In the

f ir s t model (Fig, 10.a) a feldspar megacryst experiences le ft-la te ra l

simple shear displacement along fractured surfaces parallel to the

shear direction. In the second model (Fig. 10.b) the feldspar also ex­

periences le ft-la te ra l simple shear displacement along the fractured

surfaces. However, this movement is a result of rigid body rotation

accompanied by right-lateral shear and resulting overall top-to-the-right 24

/yirn

M l Me

FIG. 9. Me and Mm planes indicate rig h t-la te ra l and le ft-la te r a l displacement respectively in sections cut normal to Me and parallel to Ml, viewed northward.

y y A

b.

FIG. 10. (a) Feldspar megacryst experiences le ft-la te ra l simple shear along fractured surfaces parallel to the shear direction. (b) Feldspar experiences le ft-la te r a l simple shear as a result of rigid body rotation accompanied by right-lateral shear resulting in overall top-to-the-right displacement in a horizontal zone. Ml Me

FIG. 11. Rotation of feldspars on axes normal to Ml requires that shear, or tectonic transport, parallel Ml. Clockwise rotation, viewed northward, of these rigid megacrysts indicates right- lateral shear (modified from Beck, 1980). 25 displacement of a horizontal zone. In the second model two apparently contradictory shear displacements result during major top-to-the-right shear in the horizontal zone. Given recognition of the well developed nature and continuity of Me shear planes, and relatively minor left-

lateral offset between megacryst fragments, the second model best fits the textures observed in the mylonite fabric. Acceptance of the

second model indicates top-to-the-east displacement during mylonitic deformation of Okanogan dome rocks.

Further support of this relationship is given by documenting

preferred rotation of megacrysts in sections parallel to Ml and lack of rotation in sections normal to Ml. Table I summarizes clockwise versus counterclockwise rotation of megacrysts in these sections

taken from traverse A. Interpretation of megacryst rotation is based on the following assymptions: 1) feldspar megacrysts are rigid bodies

in a ductiley deforming matrix; 2) the smaller portions of a megacryst rotate more than the larger portions of the same megacryst; and 3) deformation is not random in these rocks and the resulting deformation paths of several grains, or parts of grains, must be coherent or internally consistent with one another. Based on these assumptions I interpret grain rotations; Figure 6.8 pictures a fractured megacryst with arrows showing interpreted rotation direction. As seen from

Table I , clockwise rotation dominates in sections parallel to Ml whereas no preferred rotation is seen in sections normal to Ml. This

relationship illustrates that the mylonitic lineation in these rocks not" could^have formed as a result of rotation parallel to lineation as 26

proposed by Lister and Price (1978). Rotation on axes normal to Ml requires that shear, or tectonic transport, parallel Ml. Clockwise rotation, viewed northward, of these rigid megacryst indicates right- lateral shear (Beck, 1980) (Fig. 11). Minor examples of counterclock­ wise rotation are observed in sections parallel to Ml (Table I ) . Given the heterogenous nature of the parent rock, on the scale of a thin section, opposite rotation is expected of a few grains; however, the dominant rotation should reflect overall sense of shear within the mylonite zone.

Megacryst imbrication and rotation document a similar sense of shear within the Okanogan mylonite, and both indicators are present and consistent throughout the zone studied. An even more pervasive and consistent shear indicator is interpreted from the Mc-Mmplanes and their resulting asymmetry.

From the discussion above i t is clear that Mm planes are short and discontinuous relative to Me, although Mm planes deform Me. Me, in turn, deforms Mm. Therefore, these planes were active simultaneously for at least a portion of their movement histories; apparent shear on each respective surface is pictured in Figure 9. Me shows apparent rig h t-lateral displacement whereas Mm shows apparent le ft-la te ra l dis­ placement. The interpretation of simultaneous movement is also supported by the parallel trend of Ml and Ml* observed in the fie ld .

In summary, shear surfaces Me and Mm moved at the same time, yet given apparently opposite senses of shear. Therefore the major plane of shear must be identified to determine an overall direction of 27 shear during mylonitic deformation of these rocks.

I propose Me as the major plane of shear, proceeding from the following observations: 1) Me is more continuous than Mm on both mesoscopic and microscopic scales; 2) Me contains a more continuous lineation than Mm; 3) small measurable offset of megacrysts along Mm indicates minor movement occurred along Mm whereas no parts of mega­ crysts can be matched along Me; the inference here is that displacement along Me is greater than the field of view; and 4) offset of mega­ crysts along Mm is invariably contained between Me planes, and can therefore not account for major shear of the mylonite. Accepting the proposal that the major plane of shear parallels Me, the interpreted shear couple shows rig ht-lateral displacement, viewed northward, to top-to-the-east displacement during dormation of the Okanogan mylonite zone.

The Me and Mm planes discussed above are similar to mylonitic textures described in the literature for other areas. Me, representing major mylonitic fo liatio n , corresponds to: "C" surfaces of Berthe et al (1979a, 1979b); mylonitic fo liatio n , Sm, of White et al (1980); and foliation, S, of Platt and Visser (1980). In each of the above areas this plane of mylonitic fo liatio n . Me, C, Sm, and S, is a plane of major movement, parallel to the overall shear couple. Mm planes, sharp planar discontinuities cutting across mylonitic foliation at an acute angle, are similar to "S" surfaces of Berthe et al (1979a,

1979b) and crenulation zones of P latt and Visser (1980), and may be confused with shear bands, Ss, of White et al (1980). Figure 12 i l ­ lustrates the relationship of these surfaces. 28

Me (Honsenl C C^crtKe+al) SmCWKite d"ol) ^ S \ Visser)

5(BeHht ciaj) ^ Ss et al'i iHansen)

FIG. 12. "Crenulation zone" of Platt and Visser is similar to Mm (Hansen) and Ss (White et al) depending on the conditions of mylonitic deformation (see Platt and Visser, p. 406-408).

CCBerthe et al)

Mf»\ (Hansen) ScBerthe et ol )

FIG. 13. Comparison of Mm (Hansen) and S (Berthe et a l). I f the crenulation zone is defined as the sharp discontinuous surface, Mm, le ft-la te r a l offset is observed; i f the crenulation zone is defined as a broad zone, for example a mica lath , the zone shows right- lateral displacement, or clockwise rotation.

(Me ; Hansen)

FIG. 14. Comparision of Mm (Hansen) and Ssj and Ss2 (White) 29

Me and Mm surfaces of this paper and C and S surfaces of Berthe et al (1979a, 1979b) are similar in their relationship to one another, and to the interpreted direction of shear. However, an important difference exists in the pairs of surfaces. According to Berthe et al, their S surfaces contain the maximum extension direction of the deformed minerals, and correspond to the XY plane of the fin ite strain ellipsoid during the in itia l stages of deformation. Whereas Berthe et al define S surfaces as zones of elongation, or minor right-lateral shear viewed northward. Mm surfaces of Okanogan mylonites are clearly planes of left-lateral shear, viewed in the same orientation. I believe this apparent difference reflects the scale of observation

(Fig. 13). I f the crenulation zone is defined as the sharp discon­ tinuous surface. Mm, the apparent offset along that surface should be le ft-la te ra l relative to the other structures as in Figure 13. I f the crenulation zone is defined as a broad zone, for example a thick mica lath (Berthe et a l, 1979a, 1979b), the zone shows right-lateral shear or clockwise rotation as pictured in Figure 13. Offset on the sharp discontinuous surfaces is internally consistent with deformation within the zone between these sharp surfaces; the fabric interpre­ tation presented in this paper is consistent with the fabric inter­ pretation of Berthe et al (1979a, 1979b). Both surfaces. Mm and S, rotate toward mylonitic fo liatio n , decreasing the acute angle between the crenulation zone and mylonitic foliation (Berthe et al, 1979a,

1979b; Platt and Visser, 1980). 30

Mm in Okanogan mylonites and Ss of White et al (1980) represent apparently opposite sets of shear bands which develop during progressive mylonitic deformation (Fig. 14) (see P latt and Visser, 1980; p. 406-

408 for discussion; White, 1979a). Before accepting a kinematic interpretation of petrofabrics developed during mylonitic deformation, a worker must understand which fabrics are present in the rocks analyzed; i t may be most valuable to check several types of textural evidence for consistent interpretation throughout a zone of myloniti- zation.

In summary, the Okanogan mylonite formed in a shear zone as defined by Ramsey and Graham (1970); deformation of the Okanogan dome was dominated by progressive simple shear and formed during eastward displacement of upper plate rocks with respect to lower plate rocks as shown by: 1) offset of pegmatites; 2) stretching of schistose in­ clusions; 3) imbrication of feldspars, the domino effect; 4) clockwise rotation of feldspar megacrysts, viewed northward, on axes normal to mylonitic lineation; and 5) the angular relationship of Me and Mm sur­ faces, and their resulting asymmetry.

CHEMISTRY

Structural elements as well as metamorphic conditions define the environment of mylonitic deformation, and hence dome evolution. As discussed below coexisting feldspar mineralogy indicates that the

Okanogan mylonite formed under middle-greenschist facies conditions. 31

In order to evaluate metamorphic conditions of mylonitization I

studied chemical changes accompanying progressive mylonitization of a

homogeneous granodiorite. I collected samples for whole-rock major-

element analysis and oriented samples for detail mineral analysis

along traverses through the Okanogan mylonite zone (Fig. 3). Samples

for whole-rock chemical analysis were collected at alternate stations;

oriented samples were collected at every station. Whole-rock major-

element analyses should record macroscopic chemical changes through

the mylonite zone and microprobe analyses should record detailed

chemical trends within specific minerals; biotite and feldspars were

analyzed.

Whole-rock major-element analyses were determined by X-ray

fluorescence spectrometry. Mineral analyses were performed on a

Cameca computer automated microprobe. Analyses were performed on wavelength spectrometers with absorbed current of 5.5 nA. Oxide weight percent values and normative minerals are calculated from

normalized atomic concentrations. Normalized element and oxide values

for b io tite are anhydrous.

Whole-rock major-element analyses and specific gravity deter­ minations (Table I I ) indicate that no major chemical changes took

place with progressive mylonitization of megacrystic granodiorite;

thus, on a macroscopic scale mylonitization involved constant volume

and isochemical deformation. 32

FELDSPAR MINERALOGY

Although progressive mylonitization of megacrystic granodiorite

is isochemical on a macroscopic scale, feldspars on a microscopic

scale exhibit important chemical trends. Feldspar grains show signs of both b rittle and ductile deformation; fracturing and breakage

indicate dominantly b rittle processes (Lister and Price, 1978; Watts and Williams, 1979), and segregation bands with associated kinking and microfracturing indicate dominantly ductile processes (Hanmer, 1981,

1982). Very fine new grains develop along the borders of feldspar megacrysts and original grains become progressively rounded and smaller; this fine-grained recrystallization may involve both dis­

location movement and strain-enhanced diffusion (Allison et al, 1979;

Barnett and Kerrich, 1980; Etheridge and Hobbs, 1974; Kerrich et a l,

1980; White, 1976). The goal of this portion of my study is to record

chemical trends across such recrystallized feldspar grains and to use

the percent albite component of coexisting recrystallized feldspars

to estimate a temperature of their recrystallization during myloniti­ zation (after Stormer, 1975; Powell and Powell, 1977).

Tables I I I , IV, and V record major-element microprobe analyses and normative Ab, An, and Or calculations from plagioclase ( I I I ) and orthoclase (IV) megacryst cores to rims, and very fine-grained re­ crystal lization surrounding host grains, and coexisting recrystallized

feldspar pairs (V). Ab, An, and Or components, of plagioclase and

orthoclase megacryst cores and fine-grained recrystallized grains, are

plotted on a ternary diagram in Figure 15. From core to rim to 33 recrystallized matrix grains, plagioclase gains Ab component and loses

An and Or component. Kerrich et al (1980) and Hanmer (1982) report similar trends in mylonitized plagioclase. Orthoclase grains gain Or and lose Ab component in traverses from core to rim to recrystallized matrix. This change is similar to that for orthoclase reported by

Kerrich et a l, but in contrast to that of Hanmer who reports a gain in

Ab component in mylonitized orthoclase.

Given that diffusion of alkalies is an important process in feldspar recrystallization during mylonitic deformation (Allison et a l,

1979; Hanmer, 1981, 1982; Kerrich et a l, 1980) the chemical composition of coexisting recrystallized feldspar pairs ought to "lock in" in­ formation on the environment during mylonitization. The proportion of Ab component in coexisting feldspar pairs should given an estimate of temperature of mylonitization (Stormer, 1975; Stormer and Whitney,

1975; Whitney and Stormer, 1975). The assumption that diffusion plays an important role in feldspar recrystallization during mylonitization is reasonable considering the intimate interaction of dislocation mobility and diffusion during mylonitization (Allison et al, 1979;

Barnett and Kerrich, 1980; Beach, 1980; Brodie, 1980; Etheridge and

Hobbs, 1974; Herrich et a l, 1980; White, 1976). Increase in shearing stress markedly increases diffusion and reaction rates for mineralogi- cal changes (Dachille and Roy, 1964) and rates of deformation by dislocation movements (Tullis et al, 1973). In essence, dislocation deformation and diffusion metamorphism speed a system toward chemical equilibrium with its environment. 34 AN

t u fine-grained recrystallizedgrains

• plagioclase cores orthoclase cores

AB OR

FIG. 15. Ternary diagram of Ab, An, and Or in plagioclase and orthoclase megacryst cores and fine-grained recrystal 1 ized matrix

00 500

80

5 K bars E 40

20

20 40 60 7 o Ab in Orthoclase

FIG. 16. Percent Ab in fine-grained recrystallized coexisting plagioclase and orthoclase (graph a fte r Stormer, 1975). 35

Figure 16 Is a plot of percent Ab in fine-grained recrystallized coexisting plagioclase and orthoclase taken from rocks throughout the mylonite zone along traverse A. Coexisting feldspar pairs form a tight cluster of points just below 400*C on the 5 kbar plot generated by

Stormer (1975). I chose to plot the data on the 5 kbar plot based on

Sibson's (1977) classification of fault rocks and mechanisms. Al­ though the 400®C curve of Stormer (1975) is dashed and therefore of lesser confidence, the coexisting feldspar pairs plot well below this curve. This temperature of less than 400°C is an agreement with

Kerrich et al (1980) who report similar chemical trends, and compositions, in mylonitized feldspars. They determined a mylonitization temperature of approximately 250*C ± 30®C as determined by fluid inclusion fillin g temperatures in syntectonic microveins, from 518 0 in quartz-ilmenite of +150/100, and from mineral s ta b ility c rite ria .

In summary, feldspar grains of granodiorite in the Okanogan mylonite show transitional behavior between ductile and brittle processes and exhibit consistent chemical trends with mylonitic de­ formation. Plagioclase gains Ab, and loses An and 04 during mylonitic recrystallization whereas orthoclase gains Or and loses Ab. A plot of percent Ab component in coexisting recrystallized feldspar pairs indicates a temperature of less than 400®C during mylonitization. This temperature corresponds to middle-greenschist facies conditions.

BIOTITE CHEMISTRY

Biotite from Me and Mm surfaces throughout the mylonite zone along 36 traverse A were analyzed for major elements to determine whether chemical variations exist between Me and Mm biotite. The presence of two distinctly different biotite compositions may indicate strain-

induced chemical potential gradients within and across shear zones

(Kerrich et a l, 1980). Anhydrous, normalized atomic concentrations, oxide weight percentages, and Fe: Mg:Ti ratios are given in Table VI.

Fe:Mg:Ti ratios of Me and Mm b iotite are plotted in Figure 17. Slight

variations in bio tite chemistry exist. However, no consistent trends

in chemical change are present in comparison of: 1) b iotite core to

rim variations; 2) Me versus Mm biotites; 3) increased mylonitic

deformation of Me biotite; or 4) increased mylonitic deformation of Mm

b io tite.

No major chemical differences exist in the cores of apparently

undeformed biotite laths and finely recrystallized biotite of Me and

Mm. Therefore, either all biotite recrystallized to a similar com­

position, or b io tite did not recrystal1ize during mylonitization. The

fact that there is no detectable chemical difference between Me and

Mm b io tite supports the hypothesis presented above that Me and Mm were

active simultaneously.

I conclude from the data presented above that the mylonite within

the southwest sector of Okanogan dome formed under middle-greenschist

facies conditions during eastward displacement of upper plate rocks

relative to lower plate rocks. The mylonitic fabric is superimposed

on an e a rlie r regional high-grade metamorphism and associated defor­

mation, and is cut by a later low-grade b rittle deformation and T, 37

Fe Mg

FIG. 17. Fe:Mg:Ti ratios of Me and Mm b io tite.

TABLE I

TABLE I I : Whole rock major element analyses — inc. rnyl. def —>- — inc. myl. def. — inc. tnyl def. Analysis 2A 4A 6A 8A lOA 28 48 68 78 2C 4C 6C 8C SiOo 69.99 68.57 70.68 70.31 72.10 70.35 71.03 70.24 70.05 69.60 70.65 70.17 71.37 17.54 18.35 17.42 17.27 16.62 17.55 16.83 17.14 17.26 17.75 17.17 17.38 16.82 îlè 0.36 0.37 0.32 0.34 0.23 0.34 0.33 0.33 0.35 0.36 0.35 0.33 0.31 FeO 1.93 1.93 1.69 1.95 1.24 1.89 1.83 1.80 2.01 2.01 1.96 2.04 1.67 MnO 0.05 0.05 0.04 0.05 0.03 0.04 0.05 0.05 0.05 0.04 0.05 0.05 0.05 CaO 3.47 3.73 3.18 3.45 2.91 3.29 3.34 3.35 3.30 3.40 3.32 3.28 3.25 MgO 0.29 0.44 1.69 0.25 0.25 0.27 0.32 0.31 0.37 0.52 0.36 0.33 0.32 K2O 1.79 1.86 1.82 1.76 2.59 2.18 1.99 2.33 2.42 2.01 1.93 2.06 2.02 Na20 4.46 4.52 3.09 4.47 3.88 3.96 4 i l l 4.31 4.03 4.20 4.04 4.22 4.09 P2O5 0.13 0.17 0.08 0.16 0.11 0.13 0.16 0.14 0.16 0.12 0.17 0.15 0.10

TABLE III: Plagioclase -----increasing myl cm1tic defc>rmatio1------—> core rim rxl rxl core rim rxl rxl core rim rxl Analysis 66 67 68 76 77 78 81 85 92 91 89 87 Si 20.925 20.903 21.255 22.109 20.640 20.842 21.052 20.756 20.791 20.852 21.421 21.782 A1 9.988 10.023 9.512 9.009 10.168 10.093 9.787 10.098 10.221 10.230 9.457 9.236 Ca 1.712 1.699 1.713 1.431 1.920 1.900 1.681 1.886 1.913 1.-99 1.361 1.445 Na 5.743 5.789 5.970 5.449 5.733 5.575 5.935 5.726 5.352 5.137 6.192 5.766 K 0.143 0.102 0.049 0.076 0.148 0.053 0.077 0.084 0.147 0.110 0.056 0.016 0 61.488 61.484 61.501 61.925 61.392 61.537 61.470 61.450 61.576 61.672 61.513 61.754

SiOo 61.41 61.37 62.44 65.11 60.47 61.13 61.83 60.86 60.93 61.09 63.09 64.15 AI2O3 24.87 24.97 23.71 22.51 25.28 25.12 24.39 25.12 25.41 25.43 23.63 23.08 CaO 4.69 4.66 4.70 3.93 5.25 5.20 4.61 5.17 5.23 5.47 3.74 3.97 Na20 8.69 8.77 9.05 8.28 8.66 8.43 8.99 8.66 8.09 7.76 9.41 8.76 <20 0.33 0.23 0.11 0.18 0:34 0.12 0.18 0.19 0.34 0.25 0.13 0.04 Ab 74.21 74.91 75.80 77.31 70.77 72.91 75.52 72.45 71.00 69.65 80.41 78.80 An 23.80 23.70 23.55 21.52 27.14 26.37 23.40 26.40 26.91 28.78 18.81 20.94 Or 1.99 1.39 0.66 1.17 2.09 0.73 1.08 1.15 2.09 1.57 0.78 0.26

TABLE IV: Orthoclase ■increasing mylonitic deformation core f rim rxl rxl core n m rxl rxl core rim rxl Analysis 74 72 70 69 75 36 37 40 42 44 20 24 22 Si 22 704 22. 767 22 .669 22.611 22.852 22 .706 22 728 22 818 22 .692 22 726 22 751 22 828 22 864 A1 8 273 8. 335 8.372 7.957 8 180 8 .150 8 161 7 992 8 138 8 104 8 154 8 120 7 997 Ca 0 00 0. 00 0 00 0.00 0.00 0.00 0. 00 0. 00 0.00 0 00 0 00 0 00 0 00 1 Id 0 922 0. 892 0.363 1.005 0 .228 1.097 0. 667 0.698 0.409 0, 539 1 045 0 787 0 422 K 6 548 6. 349 7.012 7.179 7.101 6.574 6 942 7 013 7.311 7 169 6 530 6 690 7 189 0 61 553 61. 657 61 .584 61 .248 61 .639 61 .473 61 502 61. 479 61 .450 61 462 61 520 61 575 61 528

SiO, 64 26 64. 54 63.93 63 .68 64 .41 64 .25 64. 13 64. 35 63 .85 64 02 64 40 64 54 64 40 AI2O3 19 87 20. 05 20 .03 19.01 19.56 19 57 19 54 19, 12 19 .43 19 37 19 58 19 48 19 11 CaO 0 00 0. 00 0.00 0.00 0.00 0 00 0. 00 0. 00 0.00 0 00 0 00 0 00 0 00 Na20 1 35 1. 30 0 .53 1.46 0.33 1 60 0. 97 1. 02 0 .59 0 78 1 53 1 15 0 61 <20 14 53 14. 11 15.50 15 .85 15.69 14 58 15. 36 15. 51 16.13 15 83 14 49 14 83 15 87

Ab 11 74 11. 65 4 .66 1.17 2 92 11 92 7. 68 6. 69 2.18 4 29 13 14 9 99 5 21 An 0 00 0. 00 0.00 0.00 0 00 0 00 0. 00 0. 00 0 .00 0 00 0 00 0 00 0 00 Or 88 26 88. 35 95 .34 98.83 97.08 88 08 92. 32 93. 31 97 82 95 71 86 86 90 91 94 79 TABLE VI : Biotite core ------» rim core ----- > rim Mm biotite. inc. myl. def. — > Me biotite inc. myl. def. —» lysis 5 9 8 25 26 27 47 14 21 46 36 24 52 4 19 39 35 23 Si 14.797 14.773 14.899 14.525 14.643 14.899 14.690 14.927 14.506 14.959 14.689 14.445 14.822 14.823 14.807 14.863 14.584 14.626 Al 7.363 7.465 7.605 7.229 7.896 8.305 6.948 8.334 7.525 7.921 7.574 7.246 6.910 8.051 7.445 7.656 7.287 7.484 Ti 0.965 0.931 0.968 0.906 0.793 0.543 1.096 0.724 0:982 0.626 0.857 1.010 0.959 0.977 0.943 1.150 1.176 0.926 Fe 6.516 6.675 5.845 6.626 6.003 5.497 7.264 6.358 6.564 6.019 7.451 6.703 7.135 5.968 6.466 6.342 7.070 6.529 Mg 6.690 6.535 6.925 7.176 6.922 7.233 6.374 6.137 6.689 6.777 6.059 6.911 6.521 6.437 6.830 6.064 6.136 6.928 Mn 0.110 0.091 0.088 0.262 0.232 0.165 0.219 0.087 0.214 0.193 0.174 0.268 0.203 0.100 0.167 0.183 0.192 0.260 Ca 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Na 0.120 0.010 0.067 0.029 0.113 0.034 0.069 0.019 0.034 0.113 0.000 0.064 0.212 0.099 0.048 0.114 0.136 0.000 K 5.021 5.072 5.049 4.976 4.980 4.715 4.971 4.678 5.158 4.864 4.687 6.911 4.897 4.876 4.760 4.982 5.000 4.801 0 58.430 58.448 58.556 58.271 58.419 58.610 58.370 58.735 58.327 58.529 58.499 58.247 58.340 58.669 58.534 58.646 58.418 58.447 Si02 38.22 38.08 38.83 37.41 38.08 39.21 37.46 38.81 37.34 39.00 37.57 37.10 37.95 38.60 38.32 38.39 37.28 37.76 16.14 16.32 16.82 15.80 17.42 18.54 15.04 18.39 16.44 17.52 16.43 15.79 15.01 17.79 16.35 16.78 15.81 16.39 2 3.27 3.19 3.35 3.10 2.74 1.90 3.72 2.50 3.36 2.17 2.91 3.45 3.27 3.38 3.25 3.95 4.00 3.18 FeO 20.12 20.57 18.21 20.41 18.67 17.30 22.17 19.77 20.20 18.76 22.78 20.58 21.88 18.58 20.01 19.59 21.61 20.16 MgO 11.59 11.30 12.11 12.40 12.08 12.77 10.91 10.71 11.55 11.85 10.39 11.91 11.20 11.25 11.86 10.51 10.52 12.00 MhO 0.34 0.28 0.27 0.80 0.71 0.51 0.66 0.27 0.65 0.59 0.53 0.81 0.61 0.31 0.51 0.56 0.58 0.79 0.16 0.01 0.09 0.04 0.15 0.05 0.09 0.03 0.05 0.15 0.00 0.08 0.28 0.13 0.06 0.15 0.18 0.00 K2O 10.17 10.25 10.32 10.05 10.15 9.73 9.94 9.53 10.41 9.94 9.39 10.28 9.83 9.95 9.66 10.09 10.02 9.72

Fe 46.02 47.20 42.55 45.05 43.76 41.42 49.30 48.10 46.11 44.84 51.86 45.84 48.82 44.60 45.41 46.78 49.16 45.40 Mg 47.25 46.22 50.41 48.79 50.46 54.50 43.26 46.43 46.99 50,49 42.18 47.26 44.62 48.10 47.97 44.74 42.66 48.17 Ti 6.72 6.58 7.04 6.16 5.79 4.09 I 7.44 5.48 6.90 4.67 5.96 6.90 6.56 7.30 6.63 8.48 8 18 6.44

TABLE V: Coexisting feldspars increasing mylonitic deformation Analysis 31 34 53 54 75 76 64 65 84 85 42 41 88 89 22 23 Si 22.890 20.925 23.229 20.860 22.852 22.109 22.646 20.624 22.749 20.756 22.692 23.095 22.902 21.421 22.864 21.053 Al 8.030 9.951 7.412 9.998 8.180 9.009 8.199 10.219 8.320 10.098 8.138 8.09E 8.084 9.457 7.997 9.851 Ca 0.000 1.914 0.000 1.899 0.000 1.431 0.000 2.117 0.000 1.886 0.000 1.069 0.000 1.361 0.000 1.769 Na 0.613 5.605 0.760 5.673 0.228 5.449 0.629 5.481 0.440 5.726 0.409 5.486 0.263 6.192 0.422 5.683 K 6.891 0.074 7.095 0.079 7.101 0.076 7.081 0.083 6.863 0.084 7.311 0.069 7.127 0.056 7.189 0.101 0 61.576 61.531 61.504 61.491 61.639 61.925 61.445 61.476 61.629 61.450 61.450 62.183 61.624 61.513 61.528 61.543

SiOp 64.62 61.36 65.49 61,17 64.41 65.11 63.83 60.37 64.23 60.86 63.85 68.23 64.54 63.09 64.40 61.79 AI2O3 19.23 24.76 17.73 24.87 19.56 22.51 19.61 25.38 19.93 25.12 19.43 20.30 19.33 23.63 19.11 24.53 CaO 0.00 5.24 0.00 5.20 0.00 3.93 0.00 5.78 0.00 5.17 0.00 2.95 0.00 3.74 0.00 4.85 NapO 0.89 8.48 1.11 8.58 0.33 8.28 0.91 8.27 0.64 8.66 0.59 8.36 0.38 9.41 0.61 8.60 KpO 15.25 0.17 15.68 0.18 15.69 0.18 15.65 0.19 15.19 0.19 16.13 0.16 15.75 0.13 15.87 0.23 Ab 7.71 72.66 4.04 72.65 2.92 77.31 4.87 69.57 5.70 72.45 2.18 81.94 3.34 80.41 5.21 74.11 w An 0.00 26.33 0.00 26.27 0.00 21.52 0.00 29.29 0.00 26.40 0.00 16.97 0.00 18.81 0.00 24.50 vo Or 92.29 1.01 95.96 1.08 97.08 1.17 95.13 1.14 94.31 1.15 97.82 1.10 96.66 0.77 94.79 1.39 40 associated deformation, and is cut by a later low-grade brittle de­ formation and associated chloritization (Snook, 1965; Goodge, 1983;

Goodge and Hansen, in press; Hansen, 1981).

CONCLUSIONS

Okanogan dome displays many characteristics suggestive of diapiric emplacement: 1) smooth sharp outer boundaries; 2) strong foliation developed in dome rocks generally parallels the dome boundaries; and

3) maximum deformation within the dome is present at the margins and disappears toward the dome interior (see Pitcher and Berger, 1972).

Despite these sim ilarities with documented examples of dispiric rise, the Okanogan dome mylonite displays an important difference. Although the mylonitic foliation parallels the dome boundaries, the accompanying lineation is unidirectional throughout the dome, regardless of the dip of the foliation, and does not form steeply plunging radial lineations as in documented examples of diapiric emplacement.

The relationship of mylonitic foliation parallel to the dome margin, together with unidirectional lineation are best explained with a two- phase model of dome evolution. A sub-horizontal mylonite zone formed as an intracontinental shear zone during eastward displacement of upper- plate rocks, in a direction parallel to mylonitic lineation. Granitoid rocks deformed in the shear zone included biotite-megacryst granodiorite of this study which intruded amphibotite-grade crystalline rocks.

Later doming gently warped the 1 to 1.5 km thick mylonite zone causing mylonitic foliation to parallel the late dome borders, yet preserved 41 the earlier-formed unidirectional mylonitic lineation. This doming was accompanied by brecciation and chloritization which overprints the mylonitic fabric, and probably occurred during the documented

Tertiary thermal event (Armstrong et a l, 1977; Matthews, 1981;

Medford, 1975; M iller and Engels, 1975; Ross, 1974, 1975) which reset

K-Ar dates to 50 to 60 Ma within the dome boundaries (Fox et a l, 1977).

TECTONIC FRAMEWORK

The relative timing of geologic events and kinematic interpreta­ tion of mylonitic deformation of Okanogan dome rocks is strikingly

similar to the geologic and kinematic history proposed for the Monashee complex (Brown and Murphy, 1982; Read and Brown, 1981). These workers propose that the unidirectional east-west-trending mylonitic fabric

in the Columbia River fau lt zone formed during eastward displacement of

the hanging wall relative to the foot wall, following the peak of

regional high-grade metamorphism; younger motion on the Columbia River

fau lt induced gentle macroscopic folding and brecciation of the mylonitic layering. Similar-trending unidirectional mylonitic lineation and interpreted top-to-the-east displacement are also present in the

Kettle (?), Spokane (Rhodes, 1983), and Bitterroot (Hyndman, 1980) domes

(Fig. 1). In these domes b rittle deformation and associated c h lo riti­

zation are also documented as post-dating mylonitization.

Similarities in these tectonic features and their kinematic in­

terpretation suggest a related tectonic origin. Tempieman-Kluit

(1979), and Mattauer et al (1983) and Brown and Read (1983) propose 42 similar tectonic models for the northern-most Cordillera in the Yukon and the Shuswap complex respectively. These models address the per­ vasive unidirectional transverse lineation, recumbent folding, and eastward thrusting. These workers relate the mylonite zones to large- scale intracontinental alpine-type eastwardly directed shear zones resulting from the Jurassic collision of the Stikine block with the

North American craton. A Tertiary thermal event caused arching, u p lift and normal faulting. This causes brittle reactivation of parts of old ductile shear zones, exposing core rocks, and resetting K-Ar dates to

Eocene ages. The conclusions of this study are consistent with the constraints of these similar tectonic models, although no absolute dates for Okanogan events are determined.

I propose that the tectonic model of Mattauer et al (1983) be ex­ tended westward, and the tectonic models of Tempieman-Kluit (1979) and

Brown and Read (1983) be extended southward to include Okanogan dome.

The amalgamated Stikine and Quesnellia blocks (see Monger et a l, 1982) collided with the North American plate during west-dipping subduction during Late Triassic-Early Jurassic time (Tempieman-Kluit, 1979).

Tempieman-Kluit (1979) recognizes the Teslin Suture, a northwest- trending 15 km-thick zone of steeply dipping variably deformed cata- clastic and mylonitic rock, as the arc-continent suture zone in south­ western Yukon Territory. Farther south, in central British Columbia,

Struik (1981) identifies a Late Triassic- early Jurassic mylonite zone, separating allochthonous Quesnellia from the Shuswap terrain of the

North American plate to the east, marking the arc-continent suture. 43

An arc-continent suture of Late Triassic-Early Jurassic has not been identified in southern British Columbia and northern United States although Triassic oceanic metasedimentary rocks include serpentinite bodies elongate parallel to metamorphic foliation crop out west of

Okanogan dome (Rinehart and Fox, 1976) and may represent disected remains of an arc-continent suture. Lens-shaped metamorphic (?) dunite bodies are also enclosed in the high-grade paragneiss assemblage (pgn) southwest of Okanogan dome (Fig. 3) (see Goodge and Hansen, in press).

These ultramafic bodies were probably emplaced as tectonic slivers of serpentinite prior to high-grade regional metamorphism and may mark an ancient melange or suture to the west of the present Okanogan dome.

The Late Triassic-Early Jurassic arc-continent suture should lie west of, and structurally above the Okanogan mylonite. The ancient suture may be d iffic u lt to identify as the zone was metamorphosed and folded with arc-continent collision during Early to Middle Jurassic

time, and later thrust eastward over the North American plate margin

(Templeman-Kluit, 1979; Struik, 1981; Brown and Read, 1983; Mattauer et al, 1983).

During Middle- to Late-Jurassic time the North American plate began

to underplate the accretted composite arc terrain to the west. This

underplanting caused eastward thrusting of the suture zone over the

North American plate (Templeman-Kluit, 1979; Struik, 1981; Brown and

Read, 1983) and associated development of large-scale easterly directed

intracontinental shear zones within the North American plate

(Templeman-Kluit, 1979; Mattauer et a l, 1983). Examples include the 44

Monashee décollement (Brown and Read, 1983) and the Okanogan, Kettle,

Spokane, and Bitterroot dome mylonite zones.

The nature of the Late-Triassic-Early Jurassic ductile deforma­ tion of rocks in the 15 km-thick Teslin suture is markedly different from the gently-dipping, relatively thin mylonite zones of the Monashee decollement, and Okanogan, Kettle, Spokane, and Bitterroot domes. This difference lies in the fact that the Teslin suture marks a suture zone between arc and continent, whereas the relatively thin mylonite zones mark deep-seated shear zones contained within the North American plate.

In comparison with the Himalayan model the Teslin Suture corresponds to the Indus Suture between the collided continents, and the Monashee decollement and similar mylonite zones correspond to the Main Central and Main Boundary Thrusts (Ganser, 1964).

Eastward displacement along deep-seated intracontinental shear zones continued through Cretaceous time though timing of specific shear zones varies (Brown and Read, 1983; Mattauer et a l, 1983). Ductile movement on the Monashee decollement ended during Late Jurassic time whereas eastward displacement probably continued on a sole fau lt be­ neath the Monashee complex (Brown and Read, 1983). The Okanogan mylonite zone may be continuous with the Monashee decollement, or i t may mark the trace of a deeper sole fa u lt with movement continuing through Cretaceous time. No absolute ages are available for the

Okanogan mylonite. Final movement on intracontinental shear zones carried crystalline slabs eastward relative to the North American craton. This resulted in shortening to the east and formation of the 45

Rocky Mountain fold and thrust belt in the south and the Mackenzie

Mountains in the north (Templeman-Kluit, 1979; Brown and Read, 1983;

Mattauer et al, 1983).

Major compressional tectonics ended by Late Cretaceous-Early

Tertiary time (Hyndman, 1980), followed by Eocene extensional tectonics

(Cheney, 1980; Ewing, 1981; Read and Brown, 1981; Rhodes and Cheney,

1981). The wel1-documented Eocene thermal event caused doming of the intracontinental shear zones and their associated infractructure, development of normal faults, and resetting of K-Ar dates (Read and

Brown, 1981; M iller and Engels, 1975; Fox et a l, 1977; Brown and

Murphy, 1982; Cheney, 1980).

Southwest of the Okanogan dome southwest-dipping normal faults cut Tertiary (?) dikes (Goodge and Hansen, in press) and Eocene volcanic rocks west of the dome (Rinehart and Fox, 1976). Within the

Okanogan dome prominent north-south-trending joints cut the mylonitic rocks and are locally intruded by Tertiary (?) microdiorite dikes

(Goodge and Hansen, in press).

The documented Eocene thermal event reflects anonomously high heat flow which may result from extensive Eocene volcanism and plutonism

LaFort (1981) outlines a model for magma generation due to post­ collision basement thrusting in the Himalayas. LaFort‘s model for leucocratic magma generation provides a possible explanation for the

Cordilleran Eocene thermal event. Tectonic burial by northward underthrusting of the relatively cold and wet Tibetan sedimentary series by the relatively hot Tibetan slab caused prograde metamorphism 46 of the sedimentary rocks and release of fluids which induced partial melting of the overlying Tibetan slab to produce leucocratic magma.

Final movement on the lower-most intracontinental fault of the

Cordillera could have placed a relatively hot crustal slab over

re lative ly cold and wet sedimentary rocks to the east. This could

cause prograde metamorphism of the underplated North American sedi­ ments, resulting release of fluids, and anatexis of the overlying

crystalline slab. Tertiary siliceous magmas of north-central

Washington may have been generated in this manner and may be responsible

for the recorded Eocene high-heat flow and reset K-Ar dates. Magma emplacement could cause the documented crustal extension resulting

in rapid u p lift and development of normal faults, exposing intra­

continental mylonite zones and their associated suprastuctures now exposed.

Therefore, the Cordilleran Eocene thermal event of north-central

Washington may result, at least in part, from Tertiary production of

leucogranite by postcol1isional easterly directed basement thrusting

over cool, wet sedimentary rocks of the North American plate. Meta­ morphic release of fluids from the underlying sediments, and rise of

these fluids into the overlying plate would cause partial melting of

the overlying crustal slab.

In summary, the geologic conclusions of this study are consistent

with the constraints of tectonic models of Cordilleran evolution pre­

sented by Tempiemena-Kluit (1979), Brown and Read (1983), and

Mattauer et al (1983). The tectonic models proposed by these workers 47

are similar to Himalayan-type collision orogen. In addition, I propose that the Himalayan-type model of magma generation (after

LaFort, 1981) may be a viable model for postcollision magma generation responsible for the Cordilleran Eocene thermal event of north-central

Washington. 48

REFERENCES CITED

Allison, I., Barnett, R. L., and Kerrich, R., 1979, Superplastic flow and changes in crystal chemistry of feldspars: Tectonophysics, 53, pp. T41-T46.

Allison, I. and Latour, I . B., 1977, B rittle deformation of hornblende in a mylonite: Canadian Journal of Earth Science, 14, pp. 1953- 1958.

Armstrong, R. L ., Taubeneck, W. H. and Hales, P. Q., 1977, Rb/Sr and K/Ar geochronometry of Mesozoic granitic rocks and their isotopic composition, Oregon, Washington and Idaho: Geological Society of America Bulletin, 88, pp. 379-411.

Barnett, R. L. and Kerrich, R., 1980, Stress corrosion cracking of bio tite and feldspar: Nature, 283, pp. 185-187.

Beach, A., 1980, Retrogressive metamorphic processes in shear zones with special reference to the Lewi si an complex: Journal of Structural Geology, 2, pp. 257-263.

Beck, M. E ., J r ., 1980, Paleomagnetic record of plate-margin tectonic processes along the western edge of North America: Journal of Geophysical Research, 85, pp. 7115-7131.

Bell, I . H. and Etheridge, M. A., 1973, Microstructure of mylonites and their descriptive terminology: Lithos, 6, pp. 337-348.

Berthe, D., Choukroune, P. and Gapais, D ., 1979, Orientations preferen- tielles du quartz et orthogneissification progresseve en regime cisaillan t: 1'example due cisaillement sud américain: Bull. Mineral., 102, pp. 265-272.

Berthe, D ., Choukroune, P., and Jegouzo, P., 1979, Orthogneiss, mylonite and non coaxial deformation of granites: the example of the south Armorican shear zone: Journal of Structural Geology, 1, pp. 31-42.

Brodie, K. H ., 1980, Variations in mineral chemistry across a shear zone in phlogophite peridotite: Journal of Structural Geology, 2, pp. 265-272.

Brown, R. L. and Read, P. B ., 1983, Shuswap terrane of British Columbia: A Mesozoic "core complex": Geology, 11, pp. 164-168. 49

Brown, R. L. and Murphy, D. C., 1982, Kinematic interpretation of mylonitic rocks in part of the Columbia River fault zone, Shuswap terrane, British Columbia: Canadian Journal of Earth Science, 19, pp. 456-465.

Cheney, E., 1980, The Kettle dome and related structures of north­ eastern Washington, in: Crittenden, M. D. et al (eds), Cordilleran Metamorphic Core Complexes: Geological Society of American Memoir 153, pp. 465-484.

Crittenden, M. D ., J r ., Coney, P. J. and Davis, G. H ., 1980, Cor­ dilleran Metamorphic Core Complexes: Geological Society of America Memoir 153, 490 pp.

Dachille, F. and Roy, R ., 1964, Effectiveness of shearing in accelerating solid phase reactions at low temperature and high pressure: Journal of Geology, 72, pp. 243-247.

Davis, G. H. and Coney, P. J ., 1979, Geologic development of the Cordilleran metamorphic core complexes: Geology, 7, pp. 120-124.

Debat, P., Soula, J. C., Kubin, L. and Vidal, J. L ., 1978, Optical studies of natural deformation microstructures in feldspars (gneiss and pegmatite from Occitania, southern France): Lithos, 11, pp. 133-145.

Etheridge, M. A , and Hobbs, H. B., 1974, Chemical and deformational controls on recrystallization of mica: Contributions to Mineralogy and Petrology, 43, pp. 111-124.

Ewing, T. E ., 1981, Paleogene tectonic evolution of the Pacific Northwest: Journal of Geology, 88, pp. 619-638.

Fox, K. F ., J r ., Rinehart, C. D. and Engels, J. C ., 1977, Plutonism and Orogeny in North-central Washington--Timing and Regional Context: U. S. Geological Survey Professional Paper 989, 27 pp.

Fox, K. F ., J r ., Rinehart, C. D ., Engels, J. C ., and Stern, I . W., 1976, Age of emplacement of the Okanogan gneiss dome north-central Washington: Geological Society of America Bulletin 87, pp. 1217- 1224.

Fox, K. F ., J r ., and Rinehart, C. D ., 1971, Okanogan gneiss dome (abs.), in: Metamorphism in the Canadian Cordillera: Vancouver, British Columbia, Canada Geological Association, Cordilleran Section, Program and Abstracts, p. 10.

Gansser, A., 1964, Geology of the Himalayas, Interscience, New York; 289 pp. 50

Goodge, J. W., 1983, Reorientation of folds by progressive myloniti- zation, Okanogan dome, north-central Washington: Geological Society of America Abstracts with Programs, 15, 5, p. 323.

Goodge, J. W., and Hansen, V. L ., in press. Petrology and structure of rocks in the southwest portion of Okanogan dome, north- central Washington: Northwest Geology.

Hanmer, S. K., 1983, Microstructure and geochemistry of plagioclase and microcline in naturally deformed granite: Journal of Structural Geology, 4, pp. 197-213.

» 1981, Segregation bands in plagioclase: non-dilational en- echelon quartz veins formed by strain enhanced diffusion: Tectonophysics, 79, pp. T53-T61.

Hansen, V. L ., 1983, Kinematic interpretation of mylonitic rocks in the Okanogan dome, north-central Washington: Geological Society of America Abstracts with Programs, 15, 5, p. 323.

Hibbard, M. J ., 1971, Evolution of a plutonic complex, Okanogan Range, Washington: Geological Society of America Bulletin 82, pp. 3013- 3047.

Hobbs, B. E ., 1981, The influence of metamorphic environment upon the deformation of minerals: in Lister, G. S. et al (eds). The effect of deformation on rocks: Tectonophysics, 78, pp. 335-383.

Hyndman, D. W., in press. Petrology of Igneous and Metamorphic Rocks, 2nd Edition: McGraw-Hill Book Co., New York, N.Y.

, 1980, Bitterroot dome - Sapphire tectonic block, an example of a plutonic-core gneiss-dome complex with its detached supra- structure: Geological Society of America Memoir 153, pp. 427-443.

Kerrich, R., Allison, I., Barnett, R. L., Moss, S. and Starkey, J., 1980, Microstructural and chemical transformation accompanying deformation of granite in a shear zone at M ieville, Switzerland; with implications for stress corrosion cracking and superplastic flow: Contributions in Mineralogy and Petrology, 73, pp. 221-242

Kerrich, R ., Fyfe, W. S., Gorman, B. E. and Allison, I . , 1977, Local modification of rock chemistry by deformation: Contributions in Mineralogy and Petrology, 65, pp. 183-190.

LeForte, P., 1981, Manaslu leucogranite: a collision signature of the Himalaya a model for its genis and emplacement: Journal of Geophysical Research, 86, pp. 10545-10568. 51

Lister, G. S. and Price, G. P., 1978, Fabric development on quartz- feldspar mylonite: Tectonophysics, 49, pp. 37-78.

Mattauer, M., Col lo t, B. and Van den Driessche, J ., 1983, Alpine model for the internal metamorphic zones of the North American Cordillera: Geology, 11, pp. 11-15.

Matthews, W. H., 1981, Early Cenozoic resetting of K-Ar dates and geothermal history of northern Okanogan Area, British Columbia: Canadian Journal of Earth Sciences, 18, pp. 1310-1319.

McMillen, D. D., 1981, Structural geology and stratigraphy of a margin of the Okanogan dome, Okanogan County, Washington: Regional Implications: Geological Society of America Abstracts with Programs, 13, p. 96.

Medford, G. A., 1975, K-Ar and fission track geochronometry of an Eocene thermal event in the Kettle River (west half) map area, southern British Columbia: Canadian Journal of Earth Sciences, 12, pp. 836-843.

M ille r, F. K. and Engels, J. C., 1974, Distribution and trends of discordant ages of the plutonic rocks of northeastern Washington and northern Idaho: Geological Society of America Bulletin 86, pp. 517-528.

Monger, J. W. H ., Price, R. A. and Trempleman-Kluit, D. J ., 1982, Tectonic accretion and the origin of the two major metamorphic and plutonic welts: Geology, 10, pp. 70-75.

Pardee, J. T ., 1918, Geology and mineral deposits of the Colville Indian Reservation, Washington: U. S. Geological Survey Bulletin 677, 186 pp.

Pitcher, W. S. and Berger, A. R ., 1973, The Geology of Donegal: a Study of Granite Emplacement and Unroofing: Wiley-Interscience, N. Y ., 435 pp.

P la tt, J. P. and Visser, R. L. M., 1980, Extensional structures in anisotropic rocks; Journal of Structural Geology, 2, pp. 397-410.

Powell, M. and Powell, R., 1977, Plagioclase-alkali-feldspar geothermometry revisited: Mineralogical Magazine, 41, pp. 253-256

Ramsey, J. G. and Graham, R. H ., 1970, Strain variation in shear belts: Canadian Journal of Earth Science, 7, pp. 786-813.

Read, P. B. and Brown, R. L ., 1981, Columbia River fa u lt zone: south­ eastern British Columbia: Canadian Journal of Earth Science, 18, pp. 1127-1145. 52

Rhodes, B. P., 1983, Kinematic analysis of mylonites from Spokane dome, a metamorphic core complex in northeastern Washington and northern Idaho: Geological Society of America Abstracts with Programs, 15, 5, p. 297.

Rhodes, B. P. and Cheney, E. S., 1981, Low-angle faulting and the origin of Kettle dome, a metamorphic core complex in northeastern Washington: Geology, 9, pp. 366-369.

Rinehart, C. D. and Fox, K. P., J r., 1976, Bedrock geology of the Conconully quadrangle, Okanogan County, Washington: U. S. Geological Survey Bulletin 1402, 58 pp.

and , 1972, Geology of the Loomis quadrangle, Okanogan County, Washington: Washington Division of Mines and Geology Bulletin 64, 124 pp.

Ross, J. V., 1975, A Tertiary thermal event in south-central British Columbia: reply: Canadian Journal of Earth Science, 12, pp. 899-902.

, 1974, A Tertiary thermal event in south-central British Columbia: Canadian Journal of Earth Science, 11, pp. 1116-1122.

Sibson, R. H ., 1977, Fault rocks and fau lt mechanisms: Journal of Geological Society of London, 133, pp. 191-213.

Snook, J. R ., 1965, Metamorphic and structural history of "Colville batholith" gneisses, north-central Washington: Geological Society of America Bulletin 76, pp. 759-776.

Stormer, J. C ., J r., 1975, A practical two-feldspar geothermometer: American Mineralogist, 60, pp. 667-674.

Stormer, J. C ., J r ., and Whitney, J. A., 1975, Geothermometry in sapphire granulite: an evaluation of two-feldspar and pyroxene methods (Abstr); Trans. Am. Geophys. Union, 56, pp. 446.

Struik, L. C., 1981, A re-examination of the type area of the Devo- Mississippian Cariboo orogen, central British Columbia: Canadian Journal of Earth Science, 12, pp. 326-332.

Tempelman-Kluit, D. J ., 1979, Transported cataclasite, ophiolite and granodiorite in Yukon: Evidence of arc-continent collision: Geological Survey of Canada Paper 79-14, 27 pp.

Tullis, J., Christie, J. M. and Griggs, D. T., 1973, Microstructures and preferred orientations of experimentally deformed quartzites: Geological Society of America Bulletin 84, pp. 297-314. 53

Waters, A. C. and Krauskopf, K., 1941, Protoclastic border of the Colville batholith: Geological Society of America Bulletin 52, pp. 1355-1418.

Watts, M. J. and Williams, G. D., 1979, Fault rocks as indicators of progressive shear deformation in the Guingamp region, Brittany: Journal of Structural Geology, 1, pp. 328-332.

White, S. H ., Burrows, S. E ., Carreras, J ., Shaw, N. D. and Humphreys, F. J ., 1980, On Mylonites in ductile shear zones: Journal of Structural Geology, 2, pp. 175-187.

White, S. H ., 1979, Large strain deformation: report on a Tectonic Studies Group discussion meeting held at Imperial College, London on 14 November 1979: Journal of Structural Geology, 1, pp. 333-339.

, 1976, The development and significance of mylonites: 25th International Geological Congress, Sydney, 1, p. 143.

Whitney, J. A., Stromer, J. C ., Jr. and Smith, R. L ., 1975, Feldspar thermal histories for three post-metamorphic granites from the Georgia Piedmont: Geological Society of America Abstracts with Programs, 7, p. 549.